The study was designed to assess the efficacy of nalmefene--a third generation naloxone in the control of appetite. The initial subjects were Prader-Willi Syndrome patients. The medication was increased stepwise over a three week period to 40 mg per day. The patients were then followed for two months in the outpatient department, receiving nalmefene for two weeks, alternating with placebo for two weeks. A final week in the Clinical Center was used for careful evaluation. No effect of the medication was found on food intake.